This invention in general relates to focusing imaging systems and in particular to a digital focusing imaging system that is particularly suitable for use in such applications as symbology reading.
Bar code technology has been used for almost thirty years in a variety of industrial and retail applications to rapidly provide machine readable information about products and processes involving those products. This technology has enjoyed its success because bar coding removes human error from data acquisition and entry processes as well as being repeatable and fast.
By convention, bar codes are systematic markings that modulate surface area in predetermined ways which encode information. Early bar codes consisted of a series of bars and spaces printed or otherwise affixed to a surface. Here, information was encoded in linear fashion as an alternating series of light and dark line pairs of predetermined sizes and sequences which represented agreed upon alphabets that translated directly into human understandable form with suitable decoding means.
While bar codes may vary in their use of formal encoding/decoding schemes, all characteristically share some common properties. For example, the density or amount of information that can be represented over a given surface area depends on the ability to form and read some minimum sized mark by which information may be transferred from code to reader. The size of such a mark is obviously limited by the means by which it can be formed and the ability of the reader to "see" or resolve it; the smaller the mark the higher the density and vice-versa In earlier "linear" or 1D bar codes (actually two-dimensional structures), information was encoded along only one dimension where density depended on the width of the thinnest light-dark line pair. In emerging more elaborate 2D, or matrix codes, information is also encoded by the smallest segment used to modulate a surface area, but now along two directions.
Linear bar codes are typically "read" with laser scanners that project a narrow beam of light that is swept across the code being modulated thereby in accordance with the variations in the codes particular pattern. The modulated light reflected or transmitted (transmission code) by the code is detected, and the information carried in the modulated return beam is extracted via suitable decoding software resident in a general purpose computer of dedicated microprocessor. Laser scanning type readers are known to exist in both hand held and stationary forms.
Common hand held scanning devices include wands that directly contact the code, lasers for distant scanning, and two-dimensional photodetector arrays such as CCDs or CMOS arrays
Wands operate by projecting a small beam of radiation onto the bar code. The diameter of the beam is made small enough to be modulated by the code and sampled fast enough to generate an electrical signal from which the required information can be easily extracted. Wands are limited in application to situations where direct contact is possible and are therefore not suitable for any applications requiring finite working distances.
Hand held laser scanners are suitable where large working distances are important because the lasers used can be focused to appropriately sized interrogation spots at long distances. Typically, a laser diode is used to project a beam of radiation that is focused and scanned over a bar code area by reflecting the beam from an oscillating mirror or rotating polygon mirror. The return beam is collected by suitable optics and directed to a photodetector to generate an electrical signal for subsequent downstream processing.
Stationary laser systems are also in widespread use for a variety of non-contact applications and are widely available at cash registers in supermarkets and the like so are now commonly known even to retail customers.
Two-dimensional array based systems operate by imaging a bar code onto a CCD or CMOS array which then generates an analog signal, typically at video rates, that represents the variation in intensity of the image. The intensity variation is typically converted into digital signal form and information is extracted via look-up (LUT) tables or the like.
Common to all of the reading modalities for bar codes are the need to be able to resolve details at the level at which information is encoded (high vs. low density), the ability to read over the required working distance for a particular application (near or distant codes), and the ability to operate under available lighting conditions or to provide suitable artificial illumination so that adequate signal to noise ratios are possible (detector sensitivity and lens speed). Obviously, these requirements are related and vary with the demands imposed by a particular application and the economics of the available solutions. Approaches to the problems associated with bar code readers have appeared in the patent literature and reflect considerations related to resolving power, working distance, targeting or aiming and framing, illumination delivery, as well as others.
For example, resolving power in laser scanning systems is related to the size of the minimum waist of a laser beam, assuming a Gaussian energy profile. For maximum power, the waist needs to be smallest to read high density bar codes. Also, it is know to provide focusing optics with laser scanners to increase working distance or provide a series of working distance zones within which bar codes can be read. For example, U.S. Pat. No. 4,920,255 issued on Apr. 24, 1990 to Stephen C. Gabeler and entitled AUTOMATIC INCREMENTAL FOCUSING SCANNER SYSTEM, discloses a stationary scanning system that includes a ranging means for determining bar code position and automatically adjusting the axial separation between various elements of a lens assembly to set an appropriate focal length to control spot size.
Other patents for laser scanning systems, such as for example, U.S. Pat. No. 5,641,958 and U.S. Pat. No. 5,347,121, both to Rudeen, and U.S. Pat. No. 5,479,011 to Rudeen, et al., advocate various means for selectively adjusting the size of the aperture stop of the optics used in conjunction with the laser beam to selectively provide different working distances in accordance with different depths of field that vary with aperture stop size.
U.S. Pat. No. 5,175,421 to Richard H. Harris describes a scanning system in which spot size is controlled through the use of an asymmetric rotating mirror system that changes the optical path length over which the laser travels to a bar code.
U.S. Pat. No. 5,140,141 describes a fixed scanning laser system in which beam direction and focus are controlled via a rotating holographic disk in conjunction with a stationary polygon mirror.
Mutli-focal length lenses have been proposed to focus lasers at different working distances as, for example, those shown in U.S. Pat. Nos. 5,438,187 and 5,565,668, both to Brad R. Redderson, et al.
In U.S. Pat. No. 5,173,603 issued to Joseph M. Lindacher a scanning laser system is described in which a rotating polygon is used in conjunction with a rotating spinner that carries a plurality of spherical mirror segments to focus the laser at different working distances.
U.S. Pat. No. 5,387,786 shows a focusing CCD based bar code reader in which the negative element of a traditional plus, minus, plus form of zooming system is displaced along the optical axis to focus in accordance with distance measurements provided via ranging system.
As described in U.S. Pat. No. 4,710,615 issued to Thomas J. Meyers, CCD's and light emitting diodes are placed along a rotating disc member at various locations to provide two-dimensional reading capability.
In U.S. Pat. No. 4,782,219 issued to David J. Crater, a system and method is disclosed by which a bar code is illuminated by dispersing a laser beam with diffusing material placed between the source and detector to enhance the readability of information embedded in bar codes by filling the relatively larger aperture of the detector with diffuse illumination that would otherwise not contribute to useable signal.
A diffuse illumination system is described in U.S. Pat. No. 5,585,616 issued to Donald L. Roxby, et al.. Here, light sources are used behind a diffuser to create illumination used in a fixed CCD imaging based system to enhance the readability of specularly reflecting surfaces on which bar codes have been applied.
Commercially available hand held fixed focus CCD based imaging type bar code readers have been marketed, but are limited in use to fixed working distances.
While many approaches have been used to solve bar code problems related to resolving power, working distance and the provision of adequate signal levels, there still remains a need for reader modalities that offer the convenience of hand held operation and appreciable working distance for use in decoding not only the well-entrenched linear bar code but the emerging matrix or 2D forms as well.
Accordingly, it is a primary object of the present invention to provide a focusing imaging system that is particularly suitable for use in a hand held bar code reader that is capable of reading linear both high- and low-density and 2D bar codes over an appreciable working distance.
It is another important object of the present invention to provide a focusing imaging system that utilizes a continuously rotating focusing disk for providing a plurality of focusing zones from which one may be selected as best while the disk continues to rotate.
It is another object of the present invention to provide a focusing objective lens system for use in resolving 2D and linear bar codes over a working distance that at least in part overlaps.
It is still object of the present invention to provide a hand held bar code reader for reading linear and 2D bar codes in low ambient lighting conditions.
It is yet object of the present invention to provide a hand held bar code reader that has omnidirectional reading capability.
It is another object of the present invention to provide an omnidirectional hand held bar code reader having an optical system that may be tilted through an appreciable predetermined angle with respect to normal incidence and still be able to resolve 2D and linear bar codes.
Still another object of the present invention is to provide a hand held bar code reader having a through the lens (TTL) targeting system by which the reader and its angular field of view with respect to a bar code may be set to assure that the bar code is within the viewable area and working distance of the reader.
Other objects of the invention will, in part, appear hereinafter and will, in part, be obvious when the following detailed description is read in connection with the accompanying drawings.